The heat-strengthening of yarn spun from optically anisotropic melt-forming polyesters has been known for some time (see Luise U.S. Pat. No. 4,183,895). While the optimum temperature and period of heating will vary according to the chemical nature of the polymer and its molecular weight, it has been a common objective to reduce both the time and temperature required for heat strengthening and yet obtain the best properties. To this end, it has been proposed to coat the as-spun fiber with certain inorganic compounds before heat treatment (See Eskridge et al. U.S. Pat. No. 4,424,184). U.S. Pat. No. 4,574,066 suggests the incorporation of a catalytic quantity of a potassium salt into a polymerization system for production of poly (6-oxy-2-naphthoate-4-oxybenzoate), followed by the extrusion of the polymer into fiber and subsequent heat treatment. This procedure is said to provide accelerated heat strengthening.
Anisotropic melt-forming copolyesters from chlorohydroquinone diacetate, 4,4'-dihydroxybiphenyl diacetate and a mixture of isophthalic and terephthalic acids are known in the art. (See Siemionko U.S. Pat. No. 4,412,058). The present invention provides an improvement in the process of heat strengthening fiber of this copolyester as well as other fibers formed from liquid crystalline polyesters.